We plan to continue our investigation of the electrophysiological events that give rise to abnormal rhythms of the heart. We will use intracellular recording via microelectrodes, studying canine cardiac Purkinje fibers and strips of canine coronary sinus, isolated and mounted in a fast perfusion system. We plan to extend our study of electrogenic sodium extrusions in Purkinje fibers, sodium-loading such fibers by exposure to potassium-free solutions and re-activating the sodium pump by exposure of the fibers to potassium-containing solutions. We plan to study the onset of the effects of pump inhibition by acetylstrophanthidin and to study the kinetics of sodium extrusion in preparations only partially inhibited by acetylstrophanthidin. We also plan to determine the exact quantitatve dependence of sodium extrusion on external potassium concentration and on internal sodium concentration. These studies will use the voltage-clamp technique to measure the electrogenic sodium current directly. We plan to complete our study of post-overdrive suppression of triggered arrhythmias in canine coronary sinus and to embark on a systematic study of the factors that determine the resting potential in such fibers as well as the relationship of the resting potential to the appearance of after-depolarizations and to triggerability.